This invention concerns a process for producing fluid (gaseous and liquid) hydrocarbons by hydropyrolysis of solid carbonaceous materials such as coal, lignite or peat.
The oldest process for treating these raw materials in order to obtain liquid or gaseous hydrocarbons is pyrolysis. This technique suffers from the disadvantages of a low yield of fluid hydrocarbons, poor quality, and a high yield of solid residue difficult to upgrade except in the case of foundry coke.
Numerous improvements to this basic principle have been proposed.
First of all, it has been proposed to improve the quality of hydrocarbons produced by pyrolysis by means of catalytic hydrogenation.
An improvement of the quality and the yield of fluid hydrocarbons has also been claimed, resulting from the use of a quick heating of the carbonaceous material (flash pyrolysis) followed with quenching in order to avoid the polymerization of the formed unsaturated hydrocarbons. U.S. Pat. Nos. 4,085,030, 4,141,794 and 4,229,185 describe a manner of using such a technique by contacting the carbonaceous material with a recycled solid residue previously heated by partial combustion.
Finally, it has been shown that the presence of hydrogen under pressure, in the course of a quick pyrolysis (flash hydropyrolysis) of the carbonaceous material, substantially decreases the amount of solid residue and provides higher proportions of light aliphatic and aromatic hydrocarbons. Many patents describe various modes of application of this technique. Thus, the quick heating of the carbonaceous material may be obtained by preheating the hydrogen stream in an oven, or by injecting oxygen into the hydrogen stream, as described in U.S. Pat. Nos. 3,960,700 and 4,225,414, or by recycling solid residue previously heated by partial combustion, as indicated in U.S. Pat. No. 3,855,070. This solid residue may also be used to manufacture hydrogen by reaction with steam, as indicated in U.S. Pat. Nos. 4,162,959 and 4,166,786.
In spite of all these improvements, the yields of fluid hydrocarbons remain low, particularly the yield of monocyclic aromatic hydrocarbons, such as benzene, toluene and xylenes (BTX), which have the greatest commercial value.
Thus, in the case of coals, the yields of liquid product range from 5 to 15% by weight of the initial carbonaceous material, whereas the production of solid residue is never lower than 40% and, in most cases, between 50 and 60%. In addition, the technological solutions to the problem of a rapid heating of the coal charge are costly and/or power consuming.